U.S. patent application number 15/725847 was filed with the patent office on 2018-09-06 for polycrystalline oxide having improved grain boundary proton conductivity.
The applicant listed for this patent is Korea Advanced Institute of Science and Technology. Invention is credited to Sung-Yoon Chung, Hyesung Kim.
Application Number | 20180251381 15/725847 |
Document ID | / |
Family ID | 63357238 |
Filed Date | 2018-09-06 |
United States Patent
Application |
20180251381 |
Kind Code |
A1 |
Chung; Sung-Yoon ; et
al. |
September 6, 2018 |
Polycrystalline Oxide Having Improved Grain Boundary Proton
Conductivity
Abstract
Provided is a polycrystalline oxide having a chemical formula
such as the following A.sub.1-xB.sub.1-yM.sub.yO.sub.3 and having
an improved grain boundary proton conductivity as an oxide having a
perovskite structure. Through the present invention, the
conductivity and chemical stability of proton conducting oxide may
be improved.
Inventors: |
Chung; Sung-Yoon; (Daejeon,
KR) ; Kim; Hyesung; (Daejeon, KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Korea Advanced Institute of Science and Technology |
Daejeon |
|
KR |
|
|
Family ID: |
63357238 |
Appl. No.: |
15/725847 |
Filed: |
October 5, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C01G 25/02 20130101;
C01F 11/02 20130101; C01G 99/006 20130101; C01P 2004/04 20130101;
H01M 8/1246 20130101; C01F 17/30 20200101; C01P 2002/34 20130101;
C01P 2004/80 20130101; C01P 2006/40 20130101; C01P 2002/36
20130101; C01G 25/006 20130101 |
International
Class: |
C01F 11/02 20060101
C01F011/02; C01G 25/00 20060101 C01G025/00; C01G 25/02 20060101
C01G025/02; C01G 99/00 20060101 C01G099/00; H01M 8/1246 20060101
H01M008/1246 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 3, 2017 |
KR |
10-2017-0027441 |
Claims
1. A polycrystalline oxide having the following chemical formula as
an oxide of a perovskite structure and having an improved grain
boundary proton conductivity: [Chemical Formula]
A.sub.1-xB.sub.1-yM.sub.yO.sub.3 wherein, A is an element having a
valence of 2+, B is an element having a valence of 4+, M is an
element having a valence of 3+, 0<x.ltoreq.0.1, and
0<y.ltoreq.0.2.
2. The polycrystalline oxide of claim 1, wherein A is any one
element selected from barium (Ba) and strontium (Sr), or a mixture
to which these elements are added.
3. The polycrystalline oxide of claim 1, wherein B is any one
element selected from cerium (Ce), zirconium (Zr), and praseodymium
(Pr), or a mixture thereof.
4. The polycrystalline oxide of claim 1, wherein M is any one
element selected from scandium (Sc), gallium (Ga), yttrium (Y),
indium (In), neodymium (Nd), promethium (Pm), samarium (Sm),
europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy),
holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb) and
lutetium (Lu) or a mixture thereof.
5. The polycrystalline oxide of claim 1, wherein the
polycrystalline oxide has a grain boundary structure in which an
amorphous layer is removed.
6. The polycrystalline oxide of claim 1, wherein the
polycrystalline oxide has a barrier energy (activation energy)
value of 0.65 eV or more caused by proton conduction.
7. The polycrystalline oxide of claim 1, wherein the
polycrystalline oxide has a proton conductivity of
7.times.10.sup.-3 S/cm or more at 500.degree. C.
8. The polycrystalline oxide of claim 1, wherein the
polycrystalline oxide comprises BaCO.sub.3 in an amount of less
than 3% when being reacted with carbon dioxide or water.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims priority to Korean Patent
Application No. 10-2017-0027441 filed Mar. 3, 2017, the disclosure
of which is hereby incorporated in its entirety by reference.
TECHNICAL FIELD
[0002] The present invention relates to a polycrystalline oxide
having improved grain boundary proton conductivity, and more
particularly, to a polycrystalline oxide, which removes an
amorphous layer present at the grain boundary and facilitates
movement of protonic defects by adjusting the composition.
BACKGROUND ART
[0003] A solid oxide fuel cell (SOFC) is composed of a structure in
which a solid electrolyte is disposed between two electrodes (a
fuel electrode and an air electrode). Hydrogen injected into a fuel
electrode is separated into a hydrogen ion and two electrons, and
the electrons move to an air electrode through an external circuit.
In the air electrode, oxygen injected and electrons entering
through the external circuit meet with each other and are separated
into oxygen ions, and then the oxygen ions move toward the fuel
electrode through a solid electrolyte. The moved oxygen ions are
reacted with the hydrogen ions in the fuel electrode to form water
which is a final byproduct.
[0004] The SOFC has various advantages such as ability to use
various fuels together with eco-friendly characteristics in which
water is produced as a reaction byproduct and a high energy
conversion efficiency. However, the thermal deformation of a
material occurring due to the high operation temperature
(800.degree. C. to 1,000.degree. C.), low durability, and a late
start-up time demand that a fuel cell be developed in a direction
that lowers the operation temperature. In order to lower the high
operation temperature, studies on a proton conducting solid oxide
fuel cell (proton ceramic fuel cell, PCFC) using protonic defects
which exhibit high mobility even at low temperature instead of
oxygen ions have been actively conducted.
[0005] The PCFC may effectively decrease low durability and thermal
deformation of a material, and the like, which were problems in the
SOFC through a low operation temperature between 400.degree. C. and
600.degree. C. Further, unlike the SOFC, hydrogen injected into the
fuel electrode is ionized, and then immediately moves to the air
electrode through the electrolyte, so that water produced as a
byproduct is produced not from the fuel electrode, but from the air
electrode. The SOFC needs an additional process for separating
hydrogen ions and water vapor by using a condenser in order to
reuse hydrogen used in the fuel electrode, whereas the PCFC does
not need the additional process, and thus has an advantage in that
process costs may be reduced.
[0006] As a solid electrolyte used in the PCFC, a perovskite
material is generally used. Perovskite is originally a mineral name
of CaTiO.sub.3, and many oxides having an ABO.sub.3 form being a
crystal structure which is the same as CaTiO.sub.3 are generally
collectively referred to as perovskite type oxides.
[0007] Among perovskite oxides used as an electrolyte of PCFC,
BaCeO.sub.3 oxide is known as a PCFC electrolyte material which has
drawn the most attention due to a relatively high proton
conductivity and a low sintering temperature (.about.1,400.degree.
C.). However, the high grain boundary resistance and the low
chemical stability have been continuously brought up as a problem
to be solved. The grain boundary refers to two crystal boundaries
having the same structure in a polycrystalline material of a metal
or an alloy, but having different directions.
[0008] In order to solve the problems, studies for increasing the
proton conductivity and the chemical stability by adjusting the
composition of the material, such as addition of Ce together with
Zr to the B-site (Ryu, K. H. & Haile, S. M., Solid State Ionics
125, 355-367 (1999)) and substitution of Ba with Sr at the A-site
(Hung, I. M. et al., Journal of Power Sources 193, 155-159 (2009))
have been continuously conducted, but the effects thereof have been
still insufficient.
SUMMARY OF THE INVENTION
[0009] The present invention has been made in an effort to solve
the aforementioned problems of the related art, and has a subject
to remove an amorphous layer present at the grain boundary by
adjusting the composition.
[0010] An exemplary embodiment of the present invention provides a
polycrystalline oxide having an improved grain boundary proton
conductivity, which exhibits high chemical stability even under an
environment of water and carbon dioxide.
[0011] Another exemplary embodiment provides a polycrystalline
oxide having a chemical formula such as the following
A.sub.1-xB.sub.1-yM.sub.yO.sub.3 and having an improved grain
boundary proton conductivity as an oxide having a perovskite
structure.
[0012] A may be any one element selected from barium (Ba) and
strontium (Sr), or a mixture to which these elements are added.
[0013] B may be any one element selected from cerium (Ce),
zirconium (Zr), and praseodymium (Pr), or a mixture thereof.
[0014] M may be any one element selected from scandium (Sc),
gallium (Ga), yttrium (Y), indium (In), neodymium (Nd), promethium
(Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb),
dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium
(Yb) and lutetium (Lu) or a mixture thereof.
[0015] Yet another exemplary embodiment provides a polycrystalline
oxide having a grain boundary structure in which an amorphous layer
is removed.
[0016] Still another exemplary embodiment provides a
polycrystalline oxide having a barrier energy (activation energy)
value of 0.65 eV or more caused by proton conduction.
[0017] Still yet another exemplary embodiment provides a
polycrystalline oxide having a proton conductivity of
7.times.10.sup.-3 S/cm or more at 500.degree. C.
[0018] A further exemplary embodiment provides a polycrystalline
oxide including BaCO.sub.3 in an amount of less than 3% when being
reacted with carbon dioxide or water.
[0019] According to exemplary embodiments of the present invention,
it is possible to describe the effects of the polycrystalline
oxides as follows.
[0020] According to at least one of exemplary embodiments of the
present invention, the conductivity and chemical stability of
protonic defects may be improved by removing the grain boundary
amorphous layer.
[0021] However, the effects which the polycrystalline oxides
according to exemplary embodiments of the present invention can
achieve are not limited to those mentioned above, and the other
effects not mentioned will be clearly understood by a person with
ordinary skill in the art to which the present invention pertains
from the following description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] The accompanying drawings included as a part of the detailed
description to assist understanding of the present invention
provide exemplary embodiments of the present invention and explain
the technical spirit of the present invention along with the
detailed description.
[0023] FIG. 1 is a schematic view illustrating a procedure in which
the amorphous layer present at the grain boundary is removed by a
method of adjusting the composition according to the present
invention.
[0024] FIGS. 2A and 2B are scanning transmission electron
microscope (STEM) images measured in the [100] projection direction
of a polycrystalline oxide in which 10 mol % of Dy as an acceptor
is added to an ABO.sub.3-type perovskite structure having a
composition of 1:1:3.
[0025] FIGS. 3A and 3B are scanning transmission electron
microscope images measured in the [100] projection direction of a
polycrystalline oxide in which 10 mol % of Dy as an acceptor is
added to an ABO.sub.3-type perovskite structure having a
composition of 0.95:1:3 by adjusting the composition.
[0026] FIGS. 4A, 4B, 4C, and 4D are graphs illustrating the
impedance results exhibiting an effect of decreasing the resistance
of the grain boundary, which is measured in Example 1.
[0027] FIGS. 5A, 5B, 5C, and 5D are Arrhenius graphs illustrating
an effect of decreasing the barrier energy analyzed before and
after adjusting the composition in Analysis Example 2.
[0028] FIGS. 6A, 6B, 6C, and 6D are graphs illustrating XRD results
measured after a chemical stability test of the polycrystalline
oxides whose composition is adjusted, in materials to which 10 mol
% of Dy, Gd, Sm, and Y are added, respectively with respect to
CO.sub.2.
[0029] FIGS. 7A, 7B, 7C, and 7D are graphs illustrating XRD results
measured after a chemical stability test of the oxides whose
composition is adjusted and is not adjusted, in materials to which
10 mol % of Dy, Gd, Sm, and Y are added, respectively with respect
to H.sub.2O.
[0030] FIG. 8 illustrates the impedance result of a polycrystalline
oxide whose composition is adjusted, to which 10 mol % of Sm is
added, measured at 500.degree. C.
DETAILED DESCRIPTION
[0031] The terms or words used in the present specification and the
claims should not be construed as being limited as typical or
dictionary meanings, and should be construed as meanings and
concepts conforming to the technical spirit of the present
invention on the basis of the principle that an inventor can
appropriately define concepts of the terms in order to describe his
or her own invention in the best way. Accordingly, since the
exemplary embodiments described in the present specification and
the configurations illustrated in the drawings are only the most
preferred exemplary embodiment of the present invention and do not
represent all of the technical spirit of the present invention, it
is to be understood that various equivalents and modified
embodiments, which may replace these exemplary embodiments and
configurations, are possible at the time of filing the present
application. Hereinafter, a polycrystalline oxide having improved
grain boundary proton conductivity according to an exemplary
embodiment of the present invention will be described in detail
with reference to accompanying drawings.
[0032] FIG. 1 is a schematic view illustrating a procedure in which
the amorphous layer present at the grain boundary is removed by a
method of adjusting the composition according to the present
invention. Protonic defects moving between oxygen atoms through the
Grotthuss mechanism have a decreased mobility in a region in which
the continuity of oxygen is decreased as in the amorphous layer. In
a proton conductor having a general ABO.sub.3-type perovskite
structure, an amorphous layer is present at the grain boundary, but
until now, there has been no study which suggests a solution to
these problems. The present invention effectively removed an
amorphous layer present at the grain boundary by adjusting the
composition, and as a result, obtained an effect in which the
proton conductivity is improved. Through FIG. 1, the problem to be
solved by the present invention can be easily understood.
[0033] FIGS. 2A and 2B are scanning transmission electron
microscope (STEM) images measured in the [100] projection direction
of a polycrystalline oxide in which 10 mol % of Dy as an acceptor
is added to an ABO.sub.3-type perovskite structure having a
composition of 1:1:3, and FIGS. 3A and 3B are scanning transmission
electron microscope images measured in the [100] projection
direction of a polycrystalline oxide in which 10 mol % of Dy as an
acceptor is added to an ABO.sub.3-type perovskite structure having
a composition of 0.95:1:3 by adjusting the composition. The present
invention will be described with reference to FIGS. 2 and 3.
[0034] The present invention removed an amorphous layer present at
the grain boundary by changing the composition ratio of A-site
atoms in a polycrystalline oxide having an ABO.sub.3-type
perovskite structure, and as a result, an object thereof is to
improve the proton conductivity and chemical stability.
[0035] Perovskite used as a proton conducting solid electrolyte is
produced as the A site, the B site, and the M site are doped with
an element having a valence of 2+, an element having a valence of
4+, and finally, an acceptor having a valence of 3+,
respectively.
[0036] The perovskite can be expressed as a chemical formula such
as A.sub.1-xB.sub.1-yM.sub.yO.sub.3, A may be any one element
selected from barium (Ba) and strontium (Sr) or a mixture to which
the elements are added, and B may be any one element selected from
cerium (Ce), zirconium (Zr), and praseodymium (Pr) or a mixture
thereof.
[0037] M (that is, an acceptor) may be any one element selected
from scandium (Sc), gallium (Ga), yttrium (Y), indium (In),
neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu),
gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho),
erbium (Er), thulium (Tm), ytterbium (Yb) and lutetium (Lu) or a
mixture thereof.
[0038] In the present invention, a BaCeO.sub.3 system was selected
as a perovskite structure to which an acceptor is added, and a
synthesis was carried out with a general composition having 1.0 mol
of Ba and a composition in which Ba is insufficient by 0 mol to 0.1
mol. It was confirmed that as compared to the former, a perovskite
oxide synthesized with a latter composition is characterized by
being present as a solid single phase without any phase
decomposition even in a high-temperature sintering process.
[0039] Meanwhile, in the BaCeO.sub.3 oxide having a perovskite
structure to which the acceptor is added, Ba may include any one
element selected from Sr, Ca, and La or a mixture to which a
portion of these elements are added, but is not limited
thereto.
[0040] Referring to FIGS. 2A and 2B, it can be confirmed that an
amorphous layer amounting to several nanometers (nm) is present at
the grain boundary.
[0041] In order to directly confirm the effects of the present
invention, it is necessary to confirm that the amorphous layer is
removed through an atomic unit analysis. Further, there is a need
for a procedure to prove an effect in which a method of adjusting a
composition suggested by the present invention improves the proton
conductivity by comparing a resistance value of a BaCeO.sub.3 oxide
made through the change in composition suggested by the present
invention with that of a BaCeO.sub.3 oxide made by a general
method.
[0042] For this purpose, each resistance value was confirmed by
using a high-angle annular dark field (HAADF) being an electron
microscopy technique capable of performing an atomic unit analysis,
an annular bright field (ABF) scanning transmission electron
microscopy (STEM) apparatus, and an impedance analysis method
(electrochemical impedance spectroscopy) capable of measuring the
resistances in the bulks and at the grain boundaries by dividing
the resistances according to the frequency region. Further, it was
confirmed whether the removal of the amorphous layer at the grain
boundary actually decreased the barrier energy by obtaining the
barrier energy (activation energy) value when passing through the
grain boundary by using a resistance value obtained through the
impedance analysis method. The details will be described in detail
through the Example, the Test Examples, and the Analysis Example
described below.
(Example 1) (Production of Sintered Body)
[0043] Example 1 will describe a method for preparing an oxide
having an adjusted composition of an A-site atom for improving the
proton conductivity in a polycrystalline oxide having a perovskite
structure, to which an acceptor having a valence of 3+ is
added.
[0044] In order to prepare a polycrystalline oxide having a
perovskite structure, to which an acceptor having an exactly
adjusted composition is added, it is preferred to use a solid-phase
synthesis method (solid-state reaction). However, the
polycrystalline oxide may also be prepared by one or more methods
selected from the group consisting of a sol-gel method, a physical
vapor deposition (PVD) method, a chemical vapor deposition (CVD)
method, a molecular beam epitaxy (MBE) method, and a vacuum
deposition method (thermal evaporation).
[0045] The solid-phase synthesis method is usually used because the
method may exactly adjust the composition of a polycrystalline
oxide having a perovskite structure, to which an acceptor is added,
may obtain high reproducibility at a low price, and easily
mass-produces the polycrystalline oxide.
[0046] When a polycrystalline oxide is prepared by the solid-phase
synthesis method, barium carbonate (BaCO.sub.3), cerium oxide
(CeO.sub.2), and a powder in the form of an oxide (M.sub.2O.sub.3)
of an acceptor each having a valence of 3+ are mixed at a suitable
ratio, the mixture is ball milled, and then an uniaxial pressing
and a cold isostatic pressing (CIP) are performed through a square
mold to form a molded body having a square shape. Thereafter, a
hard sintered body is formed through sintering at 1,400.degree. C.
for 5 hours. A preferred sintering temperature may be 1,300.degree.
C. to 1,600.degree. C.
(Analysis Example 1) (Electron Microscopic Analysis)
[0047] Analysis Example 1 will suggest a result in which an
amorphous layer structure is directly observed at an atomic unit at
the grain boundary of barium cerate
(BaCe.sub.0.9Dy.sub.0.1O.sub.3-.delta.) to which 10 mol % of Dy is
added as an acceptor and barium cerate
(Ba.sub.0.95Ce.sub.0.9Dy.sub.0.1O.sub.3-.delta.) to which the same
amount of the same acceptor is added and in which only the
composition of Ba is adjusted among hydrogen ion conductive
polycrystalline oxides. Hereinafter, an atomic unit analysis is
performed to know whether an amorphous layer is produced at the
grain boundary of a perovskite polycrystalline oxide generated by
adjusting the composition of Ba.
[0048] A sample for observation by an electron microscope used
barium cerate to which an acceptor is added, which is manufactured
by a general solid-phase synthesis method as in the method shown in
Example 1. In order to confirm the effects of the composition
adjustment on the grain boundary conductivity, a sample of a barium
cerate oxide to which 10 mol % of Dy was added as an acceptor and a
sample of a barium cerate oxide in which the composition of barium
(Ba) was adjusted were each manufactured according to a typical
polycrystal transmission electron microscopic sample manufacturing
method. Two polycrystal samples were polished to a thickness of 100
.mu.m, and then was ultrasonically cut by a disc having a diameter
of 3 mm. The disc sample was dimpled, and then was thinly
manufactured such that the specimen could be finally observed by a
scanning transmission electron microscope through ion-milling.
[0049] The HAADF-STEM method shows a contrast of an image according
to the atomic number, and makes crystal lattices be differentiated
by atomic units because a heavy atom, that is, a high atomic number
exhibits a bright contrast. In contrast, the ABF-STEM method is a
good method to confirm the position of an atom such as oxygen
having a low atomic number, and obtains an image by taking
electrons in a region (10 to 20 mrad) in which the scattering angle
is low by using the fact that a heavy atom is more scattered than a
light atom, and makes the position of the light atom an image
through the obtained image.
[0050] When the two transmission electron microscope techniques are
utilized, there is an advantage in that it can be directly
confirmed whether there is an amorphous layer present at the grain
boundary. It can be expected that an amorphous layer having a
higher oxygen ratio than a crystalline layer may be displayed as a
dark region in the HAADF-STEM mode, and may be displayed as a
bright region in the ABF-STEM mode.
[0051] FIGS. 2A and 2B are a STEM lattice image in the [100]
direction obtained by using the HAADF-STEM mode and the ABF-STEM
mode of BaCe.sub.0.9Dy.sub.0.1O.sub.3-.delta.. Further, FIGS. 3A
and 3B are STEM lattice image in the [100] direction obtained by
using the HAADF-STEM mode and the ABF-STEM mode of
Ba.sub.0.95Ce.sub.0.9Dy.sub.0.1O.sub.3-.delta.. Since the atomic
numbers of Ba (atomic number: 56), Ce (atomic number: 58), and Dy
(atomic number: 66) are not significantly different from each
other, the position of each atom is not easily spotted in the bulk.
However, oxygen (O, atomic number: 8) has a small atomic number,
and thus has an advantage in that oxygen can be easily
differentiated through the ABF-STEM mode.
[0052] It can be seen that a portion shown as black in a blue
square portion in the HAADF-STEM image in FIG. 2A is shown to be
bright in the ABF-STEM image in FIG. 2A. The observation means that
the inside is composed of an amorphous layer including an element
having a low atomic number such as oxygen instead of a hollow
space. Meanwhile, as a result of measurement, it was confirmed that
the thickness was about 5 nm.
[0053] In contrast, when the blue square images of the HADF-STEM
image and the ABF-STEM image in FIG. 3A are compared with each
other, it can be confirmed that neat grain boundary images are
equally exhibited. The confirmation indicates that the grain
boundary has a grain boundary structure having no amorphous
layer.
[0054] The images in FIG. 2B and FIG. 3B are HAADF-STEM and
ABF-STEM images additionally measured before and after adjusting
the composition, and are results measured in various regions in
order to solve the biggest disadvantage of an electron microscope
which is restricted to a local region image. As can be confirmed
from the result, it can be seen that the phenomenon is not a
phenomenon restricted to a local region, but a phenomenon commonly
occurring throughout the entire sample.
[0055] FIGS. 4A, 4B, 4C, and 4D are graphs illustrating the
impedance results exhibiting an effect of decreasing the resistance
of the grain boundary, which is measured in Example 1.
[0056] Hereinafter, an impedance analysis was performed in order to
prove effects of the presence and absence of an amorphous layer on
the proton conductivity at the grain boundary. The impedance
analysis will be described in detail through Test Example 1.
(Test Example 1) (Impedance Analysis)
[0057] An impedance analysis method used to confirm the effects of
decreasing the grain boundary resistance, which a proton conductive
polycrystalline oxide manufactured by a method of adjusting the
composition suggested by the present invention has will be
described.
[0058] The impedance analysis method is a method of measuring an
impedance (Z) by applying minute alternating current signals having
different frequencies to a sample. The portion in the bulk and the
portion at the grain boundary have dielectric constant values
different from each other, thereby exhibiting impedance results in
different frequency bands. In general, a high frequency band
indicates a resistance value of bulk, and a low frequency band
indicates a resistance value of grain boundary. The impedance
analysis result may be exhibited through a Nyquist plot, and the
Nyquist plot is an imaged graph by taking a real value of the
impedance resistance measured as the x-axis and a negative number
of the imaginary value as the y-axis. The Nyquist plot shows the
resistance value in each region in the form of a semicircle, and
has an advantage in that the first semicircle being the highest
frequency region shows a resistance value in the bulk, the next
semicircle shows a resistance value at the grain boundary, and as a
result, only the resistance at the grain boundary can be separately
measured. The impedance analysis method can separate the resistance
of the particle and the grain boundary as follows, and thus is an
analysis method capable of providing important information in the
study of analyzing resistance characteristics of a polycrystalline
oxide.
[0059] In order to confirm the resistance reduction effect at the
grain boundary, sintered samples of barium cerate
(BaCe.sub.0.9Dy.sub.0.1O.sub.3-.delta.) to which 10 mol % of Dy was
added as an acceptor and barium cerate
(Ba.sub.0.95Ce.sub.0.9Dy.sub.0.1O.sub.3-.delta.) to which the same
amount of the same acceptor was added and in which only the
composition of Ba was adjusted were manufactured. For an exact
comparison, sintering was equally performed at the same temperature
of 1,400.degree. C. for 5 hours, a sample with a square of 10
mm.times.10 mm side and a thickness of 0.7 mm was prepared, a Pt
paste was applied on both surfaces thereof, and an impedance
analysis sample was manufactured through a heat treatment.
[0060] FIG. 4A shows Nyquist plot results of
BaCe.sub.0.9Dy.sub.0.1O.sub.3-.delta. and
Ba.sub.0.95Ce.sub.0.9Dy.sub.0.1O.sub.3-.delta. measured at
200.degree. C. The graph on the right side in FIG. 4A shows an
enlarge view of the left image. When the x-axis value of the second
arc, which shows the grain boundary resistance in the Nyquist plot,
is measured, it can be confirmed that a grain boundary resistance
value, which is 266 k.OMEGA.cm before adjusting the composition,
becomes 20.7 k.OMEGA.cm after adjusting the composition, which is
decreased by about 13 folds.
[0061] As described above, when the composition of Ba is
appropriately adjusted in the present invention, the resistance
value at the grain boundary is significantly decreased, thereby
experimentally proving that the proton conductivity is increased.
The significant decrease in resistance value can be seen to prove
that the amorphous layer present at the grain boundary becomes a
resistance component in the movement of protonic defects, and to
prove that the removal of the amorphous layer by adjusting Ba
serves to improve the proton conductivity very positively.
[0062] FIG. 4B shows the Nyquist plot results of
BaCe.sub.0.9Gd.sub.0.1O.sub.3-.delta. and
Ba.sub.0.95Ce.sub.0.9Gd.sub.0.1O.sub.3-.delta. measured at
200.degree. C., FIG. 4C shows the Nyquist plot results of
BaCe.sub.0.9Sm.sub.0.1O.sub.3-.delta. and
Ba.sub.0.95Ce.sub.0.9Sm.sub.0.1O.sub.3-.delta. measured at
200.degree. C., and FIG. 4D shows the Nyquist plot results of
BaCe.sub.0.9Y.sub.0.1O.sub.3-.delta. and
Ba.sub.0.95Ce.sub.0.9Y.sub.0.1O.sub.3-.delta. measured at
200.degree. C.
FIG. 4A shows the grain boundary resistance value decreased after
adjusting the composition, and it was confirmed that the method of
adjusting the composition could improve the proton conductivity in
the polycrystalline oxide regardless of the type of acceptor
through an additional experiment.
[0063] FIGS. 5A, 5B, 5C, and 5D are Arrhenius graphs illustrating
an effect of decreasing the barrier energy analyzed before and
after adjusting the composition in Analysis Example 2.
(Analysis Example 2) (Arrhenius Analysis)
[0064] Analysis Example 2 shows a result of analyzing a barrier
energy (activation energy, Ea) value of the movement of protonic
defects by using the resistance value at the grain boundary, which
is obtained based on the impedance result performed in Test Example
1. The Arrhenius graph is an equation showing the relationship
among the conductivity, the absolute temperature, and the barrier
energy, and may be expressed as follows.
.sigma. T .apprxeq. exp ( - E a kT ) ##EQU00001##
[0065] The Arrhenius graph has an advantage in that the barrier
energy value may be obtained through the proton conductivity values
measured at various temperatures. If the reciprocal number term of
the absolute temperature is shown as the x-axis and the logarithmic
value of the product of the proton conductivity and the absolute
temperature is shown as the y-axis, the barrier energy value may be
obtained via the slope.
[0066] FIG. 5A shows the Arrhenius graph of
BaCe.sub.0.9Dy.sub.0.1O.sub.3-.delta. and
Ba.sub.0.95Ce.sub.0.9Dy.sub.0.1O.sub.3-.delta. and the barrier
energy value in each case. When the barrier energy value of a
sample manufactured with a general composition is compared with the
barrier energy value of a sample in which the composition is
adjusted, it can be confirmed that the barrier energy value is
decreased from 0.88 eV to 0.66 eV. The following results show that
the decrease in grain boundary resistance does not result from
other factors such as a change in grain boundary size or
composition, but the movement of protonic defects is unrestrained,
and as a result, the conductivity is improved.
[0067] FIG. 5B shows the Arrhenius graph of
BaCe.sub.0.9Gd.sub.0.1O.sub.3-.delta. and
Ba.sub.0.95Ce.sub.0.9Gd.sub.0.1O.sub.3-.delta. and the barrier
energy value in each case, FIG. 5C shows the Arrhenius graph of
BaCe.sub.0.9Sm.sub.0.1O.sub.3-.delta. and
Ba.sub.0.95Ce.sub.0.9Sm.sub.0.1O.sub.3-.delta. and the barrier
energy value in each case, and FIG. 5D shows the Arrhenius graph of
BaCe.sub.0.9Y.sub.0.1O.sub.3-.delta. and
Ba.sub.0.95Ce.sub.0.9Y.sub.0.1O.sub.3-.delta. and the barrier
energy value in each case. As in FIG. 5A, it is possible to confirm
an improved proton conductivity value and a decreased barrier
energy value at the grain boundary after adjusting the
composition.
[0068] FIGS. 6A, 6B, 6C, and 6D are graphs illustrating XRD results
measured after a chemical stability test of the polycrystalline
oxides whose composition are adjusted, in materials to which 10 mol
% of Dy, Gd, Sm, and Y are added, respectively with respect to
CO.sub.2, and FIGS. 7A, 7B, 7C, and 7D are graphs illustrating XRD
results measured after a chemical stability test of the oxides
whose composition is adjusted and is not adjusted, in materials to
which 10 mol % of Dy, Gd, Sm, and Y are added, respectively with
respect to H.sub.2O.
(Test Example 2) (Chemical Stability Analysis)
[0069] Test Example 2 includes effects of removal of the amorphous
layer at the grain boundary occurring by adjusting the composition
on the chemical stability of the proton conducting polycrystalline
oxide.
[0070] It is known that barium cerate to which an acceptor is added
is reacted with carbon dioxide and water as in the following
Equation, and as a result, the performance deteriorates due to
barium carbonate (BaCO.sub.3) and barium hydroxide (Ba(OH).sub.2)
produced (Ryu, K. H. & Haile, S. M. Chemical stability and
proton conductivity of doped BaCeO.sub.3--BaZrO.sub.3 solid
solutions. Solid State Ionics 125, 355-367 (1999)).
BaCeO.sub.3+CO.sub.2.fwdarw.BaCO.sub.3+CeO.sub.2
BaCeO.sub.3+H.sub.2O.fwdarw.Ba(OH).sub.2+CeO.sub.2
[0071] In order to measure the chemical stability of a barium
cerate polycrystalline oxide having no amorphous layer at the grain
boundary, in which the composition is adjusted according to the
present invention, the phase stability was confirmed at high
temperature where carbon dioxide is supplied. In order to test the
chemical stability in an environment severer than 400.degree. C. to
600.degree. C. which is an operation temperature of PCFC, a heat
treatment was performed at 800.degree. C. for 72 hours while being
supplied with carbon dioxide, and thereafter, the chemical
stability against carbon dioxide may be confirmed through XRD
analysis. Further, in order to confirm the chemical stability
against water, the chemical stability was tested in distilled water
at 85.degree. C. for 3 hours, and then the XRD analysis experiment
was performed, and as a result, it could be confirmed that the
phase of BaCeO.sub.3 was maintained in the polycrystalline oxide
having an adjusted composition, suggested by the present
invention.
[0072] FIGS. 6A, 6B, 6C, and 6D are XRD results of the samples in
which barium cerate to which Dy, Gd, Sm, and Y are added as an
acceptor are each subjected to heat treatment at 800.degree. C. for
72 hours while being supplied with carbon dioxide. It could be
confirmed that a high chemical stability was exhibited from a
sample in which the amorphous layer at the grain boundary was
removed by the method of adjusting the composition through XRD
analysis. In the case where the degree of the phase maintained is
quantitatively shown, when the peak intensity at about 29.degree.
being a position of the main peak of BaCeO.sub.3 is defined as
100%, the peak intensity at about 24.degree. being a position of
the main peak of BaCO.sub.3 is shown to be smaller than 0.1%. In
addition, when the relative contents of BaCeO.sub.3 and BaCO.sub.3
are compared to each other by using a reference intensity ratio
(RIR) quantitative analysis program from the measured XRD
experimental result values, it could be confirmed that BaCO.sub.3
was included in an amount of less than 3%.
[0073] FIGS. 7A, 7B, 7C, and 7D illustrate XRD results of barium
cerate to which Dy, Gd, Sm, and Y are added as an acceptor,
respectively. The blue line is an XRD result of a material having
an existing composition of 1:1:3 after a chemical stability test,
and the green line is an XRD result of a sample in which an
amorphous layer is removed from the grain boundary by the method of
adjusting the composition after a chemical stability test. It can
be confirmed that in the case of the material having a composition
of 1:1:3, the phase is completely decomposed, and as a result, an
XRD peak in which BaO, CeO.sub.2, Ba(OH).sub.2, and the like are
mixed is produced, whereas it can be confirmed that a sample
manufactured by the method of adjusting the composition maintains
an orthorhombic structure.
(Test Example 3) (Full-Cell Test)
[0074] Test Example 3 includes the content that the impedance is
measured by manufacturing a full-cell including both the electrode
and the electrolyte at 500.degree. C. in order to confirm whether
the removal of the amorphous layer at the grain boundary generated
by adjusting the composition is actually generated as an effect of
improving the proton conductivity in an operation temperature
region of the proton conductor.
[0075] At the anode and the cathode of PCFC, a reduction reaction
and an oxidation reaction occur as follows.
Anode: 2H.sub.2.fwdarw.4H++4e-
Cathode: O.sub.2+4H++4e-.fwdarw.2H.sub.2O
[0076] Protonic defects generated from the anode move into the
electrolyte through the Grotthuss mechanism, and thus move to the
cathode, and the thus reached protonic defectsare reacted with
oxygen at the cathode to produce water. In the present Test Example
3, in order to confirm the performance of the proton conducting
electrolyte, a full-cell test was performed by using platinum (Pt)
used as a test electrode of the anode and the cathode.
[0077] A Ba.sub.0.95(Ce.sub.0.9Sm.sub.0.1)O.sub.3-.delta.
polycrystalline oxide sintered body to which Sm was added as an
acceptor, which was prepared by the method of adjusting the
composition suggested by the present invention was ground to 170
.mu.m, and then electrodes were produced at the anode and the
cathode by using a pulsed laser deposition (PLD) method. FIG. 8
illustrates an impedance result measured by exposing the anode and
the cathode to a wet H.sub.2 gas and the air, respectively in a
full-cell manufactured by the next method. At the center of the
intersection point with the X-axis, the left part indicates a
resistance value generated by the electrolyte, and the right part
indicates a resistance value generated by the electrode. By using a
resistance value of the electrolyte measured by the next
experiment, the proton conductivity of the entire electrolyte could
be calculated, and it could be confirmed that the value was
7.times.10.sup.-3 S/cm.
[0078] Representative exemplary embodiments of the present
invention have been described in detail, but it is to be understood
by a person with ordinary skill in the art to which the present
invention pertains that various modifications are possible in the
above-described embodiment within the range not departing from the
scope of the present invention. Therefore, the right scope of the
present invention should not be defined by being limited to the
described embodiments, and should be defined by not only the claims
to be described below, but also those equivalent to the claims.
* * * * *